Wireless Mobile & Multimedia Networking

Wireless Mobile & Multimedia Networking

• This course covers WiFi 2
• The lecturer is Dr. Tazeen Syed
• The course code is 7COM1076

Outline

• IEEE 802.11 WLAN
  • Introduction
  • Architecture
  • Services
• IEEE 802.11 MAC Layer
  • DCF, PCF
  • RTS, CTS, DATA ACK
• IEEE 802.11 Physical Layer
  • DSSS
  • FHSS
  • OFDM
  • 802.11 standards

IEEE 802.11 Wireless LAN

• It is a standard specified by the Institute of Electrical and Electronics Engineers (IEEE) for wireless LAN.
• It defines rules for transmitting electromagnetic waves.
• Factors governed by the standard include frequency, data rates, and transmission power.

IEEE 802.11 Wireless LAN - Questions

• What medium is used for wired communication?
• What medium is used for wireless communication?
• Does higher frequency give higher bandwidth, throughput, and higher data rates?
• What is the issue with higher frequency?
• What is ISM band?
  • Free to transmit
  • 2.4GHz & 5GHz
  • Highly occupied
  • High frequencies

IEEE 802.11 WLAN Architecture

• Access Point (AP)
  • Centralised access point for wireless networks
  • Connects wired and wireless networks
• Station (STAs)
  • Wireless devices that access the wireless medium
• Basic Service Set (BSS)
  • A WLAN established using an Access Point (AP)
  • Wireless devices communicate with the AP
  • The AP forwards frames to destination stations
  • The BSS is the building block of the topology

IEEE 802.11 WLAN Architecture (cont.)

• Basic Service Area (BSA)
  • The area where wireless clients remain connected to the BSS
  • If a client moves outside BSA, communication is lost
• Basic Service Set Identifier (BSSID)
  • Unique MAC address of the AP identifying the BSS
  • Associated with only one AP

IEEE 802.11 WLAN Architecture (cont.)

• Extended Service Set (ESS)
  • When a single BSS doesn't cover the desired area, multiple BSSs are connected using a wired distribution system to form an ESS
  • The ESS is a union of interconnected BSSs

IEEE 802.11 WLAN Architecture (cont.)

• Ad-hoc mode
  • Two devices connect wirelessly without infrastructure devices (e.g., a router) creating a personal hotspot
• Infrastructure mode
  • Wireless clients connect to a wireless router or an access point (AP)

Access Point Controller

• When there are multiple access points, an AP controller is needed
• The goal of multiple access points is to improve the coverage area

IEEE 802.11 Services

• AP services are provided by the distribution system
• STA services are provided by every station (including APs)

Association – Related Services

• Association - Establish initial association between station and AP
• Disassociation - Association termination from station or AP
• Reassociation - Allows transfer of association from one AP to another, allowing stations to move from one BSS to another.

Distribution of Messages withing DS

• Distribution Service - Used to exchange MAC frames from a station in one BSS to a station in another BSS
• Integration Service - Transfer of data between a station on an IEEE 802.11 WLAN and a station on an integrated IEEE 802.x LAN
• Data Delivery - IEEE 802.11 provides data transmission and reception, but is not completely reliable, similar to Ethernet.

Access and Privacy Services

• Authentication - Establishes station identity
• De-authentication - Invoked when existing authentication is terminated
• Privacy - Prevents unauthorized access to messages

IEEE 802.11 Services (table)

• Shows services offered by different providers (station, distribution system) and the purpose of each service

IEEE 802.11 Medium Access Control Layer

• LLC sublayer
• Data link layer
• MAC sublayer
• IEEE 802.1, contention-free service
• Contention service
• Point coordination function (PCF)
• Distributed coordination function (DCF)
• Physical layer
• 802.11 (FHSS, DSSS, Infrared, 802.11a, 802.11b, 802.11g, 802.11n)

IEEE 802.11 Medium Access Control

• Distributed coordination function (DCF) - Fundamental access method of IEEE 802.11; based on carrier sense multiple access with collision avoidance (CSMA/CA) with RTS-CTS mechanism.
• Point Coordination Function (PCF) - polling-based access method used to provide real-time services.

IEEE 802.11 Medium Access Control (cont.)

• PCF and DCF operate concurrently in the same BSS
• The two access methods alternate with contention-free period (CFP) followed by contention period (CP)

Inter-Frame Spacing (IFS)

• Interframe spacing refers to time intervals between successive frame transmissions denoting priority levels for medium access
• Buffer between frames to avoid interference
• Four types - SIFS, PIFS, DIFS, EIFS

Inter-Frame Space (IFS) (cont.)

• SIFS - Shortest IFS with highest priority
• PIFS - Waiting time between SIFS and DIFS
• DIFS - Used by stations in DCF mode for packet transmission
• EIFS - Longest IFS for resynchronization if a physical layer detects an incorrect MAC frame reception

MAC timing - Basic Access Method

• Illustrates the timing relationship for network access using the specific methods

Four Frame Exchange - handshaking

• Describes the handshake process for communication in a CSMA/CA network using RTS/CTS mechanism

IEEE 802.11 DCF and RTS-CTS Mechanism

• Diagram illustrating the timing of frames in a DCF network communication, including defer access mechanism and use of NAV

Hidden Node Problem

• Issues regarding undetected collisions due to the lack of visibility between nodes.
  • Illustrative diagram
  • Only receivers can identify collisions

IEEE 802.11 Media Access Control

• MAC layer functions: reliable data delivery, access control, and security

Reliable Data Delivery

• Frames used for communication between stations and access points; requests, response, re-association and authentication
• Control frames used for channel access (RTS/CTS and acknowledgments
• Data frames used for data and control information

Hidden Node Problem - Solution

• Addressing the hidden node problem; illustrative diagram
• Stations use RTS/CTS exchange to avoid collisions

Disadvantage of DCF

• High power consumption
• Hidden terminal problem not entirely solved
• Fairness issues
• Best-effort service only

MAC timing - PCF Superframe

• Diagram illustrating the timing relationship of contention periods followed by contention-free periods
• Used to describe an access method using PCF for a specific time frame
• Describe use case (optional)

IEEE 802.11 PCF MAC layer

• Diagram illustrating the timing of frames in a PCF network

IEEE 802.11 Physical Layer

• ISM band - Industrial, Scientific, and Medical band used for wireless communication, including 2.4GHz, 5.8GHz
• Technologies - Frequency hopping spread spectrum (FHSS), direct sequence spread spectrum (DSSS), orthogonal frequency division multiplexing (OFDM), infrared

IEEE 802.11 Physical Layer (cont.)

• IEEE 802.11 uses four physical layer technologies
  • FHSS, DSSS, OFDM, Infrared

IEEE 802.11 Physical Layer (cont.)

• FHSS (Frequency Hopping Spread Spectrum)
  • Simple technique where transmission hops between frequencies
  • Pre-determined hopping sequence known to transmitter and receiver

IEEE 802.11 Physical Layer (cont.)

• FHSS (Frequency Hopping Spread Spectrum)
• Operates in 2.4 GHz ISM band
• Data rates of 1 and 2 Mbps
• Minimum hop rate and minimum hopping distance details provided

IEEE 802.11 Physical Layer (cont.)

• DSSS (Direct Sequence Spread Spectrum)
  • Encoding scheme for information bits (data)
  • Uses a Pseudo-random number (PN) which are orthogonal

IEEE 802.11 Physical Layer (cont.)

• DSSS (Direct Sequence Spread Spectrum)
  • Frequency - 2.4 GHz ISM band
  • data rates - 1 and 2 Mbps
  • bandwidth of each channel - 5MHz

IEEE 802.11 Physical Layer (cont.)

• Infrared - A physical technology
  • data rates - 1 and 2 Mbps
  • Wavelength between 850nm - 950nm

IEEE 802.11 Physical Layer (cont.)

• OFDM (Orthogonal Frequency Division Multiplexing)
• digital multi-carrier modulation system
• Uses orthogonal subcarriers transmitted in parallel
• modulation scheme such as QPSK, 16QAM etc
• Efficient use of spectrum
• High throughput is achieved

IEEE 802.11 Physical Layer (cont.) - Standards

• 802.11a, 802.11b, 802.11g, 802.11n standards, specific operating frequency, data rate, modulation schemes, channel characteristics, and other details.

IEEE 802.11 Network Physical Standards

• Table summarising standards (e.g., 802.11a/b/g/n/ac/ad/af/ah) with details of frequency band, bandwidth, modulation schemes etc.

Thank you/Any Questions

• End of presentation
• Contact information for the lecturer provided.

This quiz covers the key concepts of Wireless Mobile and Multimedia Networking, focusing on IEEE 802.11 Wireless LAN standards, MAC and Physical Layer mechanisms. Test your understanding of WiFi technologies including transmission mediums and frequency factors. Prepare to delve into both theoretical and practical aspects of wireless networking.